Index: src/org/python/core/PyObject.java
===================================================================
--- src/org/python/core/PyObject.java	(revision 6040)
+++ src/org/python/core/PyObject.java	(working copy)
@@ -1599,7 +1599,7 @@
     public PyObject _isnot(PyObject o) {
         // Access javaProxy directly here as is is for object identity, and at best getJavaProxy
         // will initialize a new object with a different identity
-        return this != o || javaProxy != o.javaProxy ? Py.True : Py.False;
+        return this != o && (javaProxy == null || javaProxy != o.javaProxy) ? Py.True : Py.False;
     }
 
     /**